Mudflats • Saltmarshes • Sea level changes •
Process relationships

Mudflats

The sediments deposited on mudflats are characteristically sand-dominated at their seaward edge and mud-dominated at the landward end. The lower intertidal flats are submerged for most of the tidal cycle and are subjected to strong tidal currents and wave action. Consequently, muds are kept in suspension and sands are deposited only from the bed load. Higher intertidal flats are submerged only at high tide, when current speeds fall to zero. No bed load transport takes place but, during slack water, mud settles out of suspension. However, settlement of silt and clay particles out of the water column, to form mudflats, takes many hours even under laboratory conditions where there is no current. This process alone is too slow to account for the development of the extensive mudflats around the coasts of England and Wales. Settlement of fine sediment is, therefore, facilitated by flocculation; the process of individual grains grouping together.

A high suspended sediment concentration of relatively large particles or flocculants will result in rapid sedimentation; deposition rates of several centimetres on a single tide have been recorded and several tens of centimetres over a spring / neap tidal cycle. However, the average rate of sedimentation for a mudflat, as measured over a period of years, may not be greater than 1cm/annum, reflecting a balance between erosion and deposition over a tidal cycle as well as (over longer periods) the influence of storm events and sea level rise.

Net deposition on a mudflat is not only a matter of sediment particles settling from suspension on the surface, it also depends upon a complex of factors which increase the shear strength of the sediment after deposition and prevent the particles being re-entrained on the succeeding tide. An important process is the dewatering of the deposited particles, giving them cohesion to resist re-erosion. Biological processes, such as their invasion by algae, can bind sediment together and thus impart considerable strength. These biological processes are generally regarded as fundamental to mudflat development, so any factor (such as chemical contamination) which may impair the biological viability of the sediment must be avoided.

Wave erosion of the mudflat surface provides the ultimate control of mudflat morphology. The shear stress developed at the bed as a wave passes over the mudflat depends on wave height, wave period and water depth; shear stress increases for higher waves and shallower depths. In relation to wave activity, erosion rates are controlled by the ratio between wave shear stress and mud shear strength and the length of time the surface is affected by waves. As the mudflat surface rises due to deposition and mean water depths decrease, so the bed shear stress increases until sediment deposition is balanced by erosion; a spatial balance between deposition and erosion forces which alters for each wave event. This balance is also controlled by the length of time the mudflat is affected by a wave and here the tidal amplitude is a critical factor. A large tidal range means that the residence time for water at high tide is much shorter than that for a small tidal range. Thus, extensive fine grained mudflats tend to be located in macrotidal (tidal range >4m) rather than mesotidal estuaries (range 2-4m). The more protracted wave effects within mesotidal estuaries tends to result in sandier and lower elevation intertidal flats.